Evaluation of the Flexural Strength of Interim Restorative Materials in Fixed Prosthodontics.

STATEMENT OF THE PROBLEM
Mechanical properties of interim restorations are considered as important factors specially when selecting materials for long-term application or for patients with para-functional habits. Flexural strength is one of the most important components of these restorations.


PURPOSE
The purpose of this study was to compare the flexural strength of five interim restorative materials.


MATERIALS AND METHOD
Fifty identical samples sized 25×2×2-mm were made from five interim materials (TempSpan; Protemp 4, Unifast III, Trim, and Revotek LC) according to ADA specification #27. The specimens were stored in artificial saliva for 2 weeks and then thermocycled for 2500 cycles (5-55˚C). A standard three-point bending test was conducted on the specimens with a universal testing machine at a crosshead speed of 0.75mm/min. Data were analyzed by using one-way ANOVA and Tamhane's post-hoc tests to measure the flexural strength of temporary materials.


RESULTS
One of the bis-acryl resins (TempSpan) showed the highest, and the light polymerized resin (Revotek LC) showed the lowest flexural strength. The mean values of flexural strength (MPa) for the examined materials were as follow: TempSpan=120.00, Protemp 4=113.00, Unifast III=64.20, Trim= 63.73 and Revotek LC=47.16. There were significant differences between all materials except Trim and Unifast III which did not show any statistical significant difference.


CONCLUSION
Bis-acryl resins were statistically superior to traditional methacrylate and light-cured resins. Therefore, application of bis-acryl resins should be deliberated in patients with heavy occlusion and in cases that need long-term use of interim restorations.


Introduction
One of the most important components of fixed prosthesis is temporary restorations. [1][2] During tooth preparation and before the insertion of final prosthesis, interim restorations must be delivered to patients. In prosthodontic treatment, the importance of provisional restorations is often ignored, resulting in problematic prosthesis which cannot protect the prepared teeth and sup-porting tissues sufficiently. [3][4] The desirable restoration should provide essential mechanical, biological, and esthetic properties to become successful. [5] In order to achieve these purposes, some important characteristics including polymerization shrinkage, wear resistance, color stability, and strength of resin must be considered. [6][7] From a mechanical viewpoint, the temporary rest-orative material should be chosen according to resistance to functional loads and removal forces. [5] One of the important aspects of provisional restorations, especially in case of long-span interim prosthesis with short-height pontics and connectors, is their flexural strength. [5] The flexural strength of interim prosthesis also plays a critical role in patients with parafunctional habits, bruxism, or clenching. [8] It causes much difficulty for both the patient and clinicians to keep the interim restorations intact. Any probable breakage of the prostheses leads to tooth movement as well as functional and esthetic problems. In addition, a repairing procedure may be boring and time consuming. [9] Since the early days of temporary materials in 1930s, they have changed greatly from their first generation of acrylics and premade crown to more recent bisacryl materials and computer-aided design/computeraided manufacturing (CAD/CAM) restorations. [10] In accordance to their composition, the interim restorative materials are categorized to 4 groups; polymethyl methacrylate, polyethyl or butyl methacrylate, microfilled bisphenol A-glycidyl dimethacrylate (Bis-GMA) composite resin, and urethane dimethacrylate (light-polymerizing resins). [5,11] Poly (methyl methacrylate) (PMMA) resins are relatively inexpensive; they render good color stability, good marginal accuracy, and excellent polishability.
However, the main drawbacks of this type of resins are high polymerization shrinkage, exothermic polymerization, low strength, low wear resistance, and pulpal irritation as the result of excess free monomers. In comparison to PMMA resins, poly R′ methacrylates have low polymerization shrinkage and low exothermic reaction. However, they have limitations in clinical use such as low strength, low wear resistance, and low color stability. Bis-acryl composite resins are superior to methacrylate base resins as the result of their low polymerization shrinkage, low exothermic reaction, good wear resistance, and good strength. Nonetheless, they are ex-pensive, brittle, less polishable, and much more difficult to repair. [12] There is no interim material which can fulfill all requirements for every situation. [13][14] Therefore, clinicians always select their product based on the determinant factors such as cost effectiveness, esthetic, strength, marginal adaptability, and easy manipulation. Revotek LC after thermocycling. The null hypothesis was that there are not significant differences in flexural strength of these temporary materials.

Materials and Method
Five interim restorative materials including Unifast III, Trim, Protemp 4, TempSpan, and Revotek LC were used in this study. They were indicative of the four types of interim materials described previously. Table 1 shows the characteristics of these materials.
A Plexiglas split mold was used to make specimens of 25×2×2-mm dimensions according to ADA specification #27. [16] The interim materials were prepared according to the instructions of the manufacturing company; then they were injected to the mold. Trim and Unifast III were mixed manually; TempSpan and Protemp 4 were mixed automatically by using dispenser tip. Revotek LC was put into the mold by hand and a spatula.
A weight of 1.5 kg was fixed on the glass slab placed on the surface of the mold to remove the excess Light-cured Self-cured Self-cured Self-cured Dual-cured  Finally, data were analyzed by using one-way ANOVA and Tamhane's post-hoc tests for multiple comparisons between and within groups. For all statistical analyses, the significance level of α = 0.05 was used. Table 2 shows the mean and standard deviation of flexural strength for each examined material. They can also be considered as a useful predictor of clinical performance. [17] In order to partially simulate oral environment, the specimens were stored in artificial saliva for 2 weeks and thermocycled for 2500 cycles between 5 C and 55˚C. Then, all samples were examined by using the  [19][20] Besides all these virtues, the physical properties of composites and poly-acid modified composites may be enhanced by a protective layer over the surface of these restorative materials. [21] On the other hand, conventional methacrylate resins are of low molecular weight, mono-functional, and have linear molecules which can reduce the strength and rigidity of restorations. Additionally, if they are not polymerized under pressure, their strength will decrease due to the trapped air bubbles. [9,18,22] Poonacha et al. [23]  TempSpan, which showed the highest flexural strength in the present study, is a dual-polymerizing material that has both auto-and light-polymerizing components which can increase the degree of polymerization; whereas, Protemp 4 is an autopolymerizing resin. [8] Protemp is a bis-acryl resin with a flexible crosslinked polymer structure which improves the strength and hardness of the material. [20,26] Since it is hydrophobic, it ensures minimal water uptake and, thus, reduces the plasticizing action. [20] The rigid central structure in bis-acryls decreases the dissolution of the resin-filler particles during their immersion in saliva. [27] Kerby et al. [28], in their evaluation of mechanical properties of urethane and bis-acryl interim resin materials, concluded that bis-acryl Protemp Plus showed significantly greater flexural strength and work-offracture than other resins after 24 hours of wet storage.

Results
They stated that the monomer system of Protemp Plus could produce polymers that were less susceptible to water sorption than urethane-based resins. This result is similar to our study which showed the superiority of bis-acryl resins over the light-cure resins.
Some authors claimed that during the bending test, Trim specimens were deflected without breakage, and the maximum force recorded by the universal testing machine was measured. Other researchers also found that after different storage conditions, extreme plastic deformation without fracture occurred and this resulted in the failure of Trim specimens. [14,17,24] Sharma et al. [29] stated that the flexural strength of PMMA was comparatively better than the flexural strength of Urethane dimethacrylate (UDMA). Their result is consistent with our study. Moreover, they claimed that in order to remove the excess material during initial polymerization, UDMA specimens were taken out and then placed again in the mold for complete polymerization. This might deform the material and change its flexural strength.
One of the limitations of this study was the weak correlation between monotonic flexural strength and resistance to fatigue loading. The fatigue tests proved to be more pertinent than monotonic flexural strength.
Therefore, testing materials under one consistent load may be inadequate to provide relevant information for long-term clinical performance. [30] Additionally, the interim resin materials can also be affected by saliva, food components, beverages and interactions among them in the oral environment. [20,[31][32][33] There are no published studies to identify the appropriate mechanical properties of interim materials which can best aid the clinicians to predict the clinical performance of these materials in vivo. [34] Hence, all clinicians should be aware of various characteristics of restorations and select the suitable temporary material for each patient. [8] Conclusion According to the present study, it can be concluded that bis-acryl interim materials present higher flexural strength than methacrylate-based resins. Therefore, application of bis-acryls in patients with heavy occlusion can be considered. It seems that these interim restorations might work in long-term use. According to our results, we can also suggest the application of dualcuring temporary materials when high mechanical strength is needed.